After creating a device to sum an initializer list as an example of this new feature last time, we created a generic function to receive a function object, which worked just very similarly to Smalltalk’s inject. After creating the usual function object, overloading the () operator, we saw a much cooler way of doing that using lambdas, like this:

Pretty cool. Pretty weird too. Let’s analyze how to declare a lambda before we continue discussing about the usage of this new feature.

auto f

Auto f. Lambdas have a type, and you can explicitly declare it. The type might be quite complicated though, and it doesn’t really add any information that makes reading the code any easier, so we are better off using C++0x’s new feature, auto, which will infer the type for us. Saves a lot of typing, trust me.

auto f = [] (int a, int b)

Brackets, and then the parameters specification. Looks weird but the brackets are there just to tell the compiler “hey, an anonymous method comes here” (actually the brackets could be omitted in this case but we’ll need them later on). After that, it’s just a normal method declaration. Useless trivia: before C++0x you could have anonymous objects but not anonymous methods. Can you think where would you have an anonymous object? I think I even wrote an article about it on this blog, but I’m too lazy to search for it.

auto f = [] (int a, int b) { return a+b; }

After the lambda’s signature, which is the same as the signature of a common method, you have the body of the method. It can be as short or as long as you want, it’s just a method’s body (though for the sanity of the maintainer you’d better keep it short).

Watch out though, that’s not the end of the declaration, we’re missing a crucial piece on this lambda:

auto f = [] (int a, int b) { return a+b; }<strong>;</strong>

I’d use the blink tag, but I think it has been deprecated. Notice that last semicolon; when you declare the lambda’s body you finish with a semicolon inside, just as you would inside a normal method, but that’s not the end of the expression, for the lambda declared outside the body of that method still needs another semicolon to appraise the compiler god.

And now, we know almost everything we need to know about lambda’s syntax. Notice how the return type of the method is automagically deduced. That’s useful, but there’s something the compiler can’t deduce by itself about the return type. If you are trying to return a reference to something, you need to make this explicit, the compiler has no way of detecting if you need a copy return or a reference return (until the next draft of C++, which I heard incorporates mind reading capabilities into the compiler. You may have to wait a couple of years though). This is easy to specify, though:

auto f = [&] (int a, int b) { return a+b; };

Just add an ampersand between the brackets and the lambda will return a reference instead of a copy of whatever object you’re trying to return. How about receiving a reference instead of returning one? That’s easy, remember the signature is the same as any other method:

auto f = [] (int& a, int& b) { return a+b; };

That’s it, now we know how to use basic lambdas. You should keep in mind, though, this mythical beast has a lot more to it than just its syntax. We’ll discuss about some of it’s darkest secrets, how to use it, when to use it, how does it work. That discussion is for next time, though.

And then we said this can be improved using some new C++0x wizardry to support actions other than adding. How would we do that? Easy, we need to decouple the iteration of the list from the operation logic. We can do something like this:

We had to do some changes other than passing the operation-object into the do_something method; since the start value (zero) was hardcoded we had to remove it to really decouple the action from the iteration.

Other than creating a function object (which is the correct name for the object wrapping our operation) we don’t see any strange changes, there’s no C++0x there, but C++0x gives us a little tool which gives you the power of creating much simpler and nicer code, or to make the next maintainers’ life a living hell. That’s a discussion for other time though, now let’s take a sneak preview a lambdas, the evolution of function objects:

Note that we didn’t change anything on the method iterating the list, we just changed main! There’s a lot to talk about lambdas, so this is only an intro to the subject. Next time we’ll discuss the subtleties of the new syntax.

for (FooContainer::const_iterator i = foobar.begin(); i != foobar.end(); ++i)

could be transformed into the much cleaner

for (auto i = foobar.begin(); i != foobar.end(); ++i)

Yet we are not done, we can clean that a lot more using for range statements.

Ranged for is basically syntactic sugar (no flamewar intended) for shorter for statements. It’s nothing new and it’s been part of many languages for many years already, so there will be no lores about the greatness of C++ innovations (flamewar intended), but it still is a nice improvement to have, considering how tedious can be to write nested loops. This certainly looks much cleaner:

for (auto x : foobar)

This last for-statement, even though it looks good enough to print and hang in a wall, raises a lot of questions. What’s the type of x? What if I want to change its value? Let’s try to answer that.

The type of the iterator will be the same as the type of the vector, so in this case x would be an int:

Last time we saw how you can use C style array-initialization for C++ objects, like this:

std::vector<int> v = {1,2,3,4};

We also saw this works for may types of objects, like maps and pairs. How about custom developed objects? Yes, that’s right, you can have initilizer lists for your own objects too, and it’s quite easy. C++0x offers initializer_lists which you can use on your constructors (or any other methods, for that mater) to have C-style initialization. Let’s see an example. We already know how to sum a list of numbers using template lists and variadic templates, so let’s try adding an initializer consisting of numbers:

That’s interesting, as you can see an initializer list is actualy a template class, meaning that nested initializers can easily be defined too. Now, we have the interface, how can we access each element of the list? Let’s do the same thing I did when I found out about initilizers, let’s search for the header file.

Looks surprisingly easy (note that this is for G++ 4.something only). And it is, most of the magic happens in the compiler, so the userland code is quite straight forward. According to that header file, we could build something like this:

As you can see, the initializer lists can be used in any place an iterable container is required, as long as it’s const. There’s not much more magic to it, but we can use some more C++0x devices to make our list-adding device much more cool, for example to support different actions and not only addition. Next time, though.

PS: An important lesson from this article: don’t be afraid to look into the system headers, they won’t bite. You should never ever change them, but taking a peek can only improve your C++ knowledge.

If you did compile it with g++, you may have noticed an interesting error message:

error: in C++98 ‘v’ must be initialized by constructor, not by ‘{...}’
warning: extended initializer lists only available with -std=c++0x or -std=gnu++0x

That’s interesting. Try to compile it with g++ again, but using C++0x instead of plain C++. Magic, now it works!

Initializers lists bring the best of C to C++ world (?) by letting you use initialize any object with an initializer. And I mean *any* object, not just vectors. For example, say you have a map (a map and a bunch of other stuff):

In the last four entries we worked on a simple example, like the one I’m pasting below, of type inference with decltype, which led us to learn about delayed type declaration and decltypes with auto. This time I want to focus just on the auto keyword instead.

We saw last time how decltype can be used in a contrived way to create a local variable without specifying its type, only how to deduce the type for this variable. Luckily, that verbose method of type declaration can be summed up in the following way:

int x = 2;
int y = 3;
decltype(x*y) z = x*y;

int x = 2;
int y = 3;
auto z = x*y;

That’s right, when you are declaring local variables it’s easier and cleaner to just use auto. This feature isn’t even “in the wild” yet, so you can’t really predict what will people do with it, but it seems to me that limiting its use to local variables with a very short lived scope is the best strategy. We are yet to see what monstrosities the abuse of this feature will produce, and I’m sure there will be many. Regardless of their potential to drive insane any maintainers, its best use probably comes in loops.

In any C++ application, you’ll find code like this:

for (FooContainer<Bar>::const_iterator i = foobar.begin(); i != foobar.end(); ++i)

This ugly code can be eliminated with something much more elegant:

for (auto i = foobar.begin(); i != foobar.end(); ++i)

Looks nicer indeed, but we can improve it much further with other tools. We’ll see how the next time. For the time being, let’s see for what auto is not to be used.

When using auto, keep in mind it was designed to simplify the declaration of a variable with a complex or difficult to reason type, not as a replacement for other language features like templates. This is a common mistake:

Wrong

void f(auto x) {
cout << x;
}

Right

template <T>
void f(T x) {
cout << x;
}

It makes no sense to use auto in the place of a template, since a template means that the type will be completed later whereas auto means it should be deduced from an initializer.

After a long, long hiatus, the C++0x series are back. You may want to check where we left by reading the last posts of this series.

In the last few entries we saw how to use decltype for type inference. Object types is a problem that seems easy but gets complicated very quickly, for example when you start dealing with constness. Constness is difficult in many ways but this time I want to review how constness works with type inference. This topic is not C++0x specific as it’s present for template type deduction too, but decltype adds a new level of complexity to it.

Let’s start with an example. Would this compile?

struct Foo {
int bar;
};
void f(const Foo foo)
{
foo.bar = 42;
}

Clearly not, having a const Foo means you can’t touch foo.bar. How about this?

struct Foo {
int bar;
};
void f(const Foo foo)
{
int& x = foo.bar;
}

That won’t compile either, you can’t initialize an int reference from a const int, yet we can do this:

void f(const Foo foo)
{
const int& x = foo.bar;
}

If we know that works it must mean that s.result’s type is const int. Right? Depends.

Just as the name implies decltype yields the declared type of a variable, and what’s the declared type for Foo.bar?

That’s an interesting difference, but it makes sense once you are used to it. To make things more interesting, what happens if you start adding parenthesis (almost) randomly? Try to deduce the type of x:

void f(const Foo foo)
{
decltype((foo.bar)) x
}

If decltype(x) is the type of x then decltype((foo.bar)) is the type of (foo.bar). Between foo.bar and (foo.bar) there’s a very important difference; the first refers to a variable whilst the last refers to an expression. Even though foo.bar was declared as int, the expression (foo.bar) will yield a const int&, since that’s the type (though implicit and not declared, since the expression is not declared).

As I said, disappearing constness is not a C++0x specific problem as it may occur on template type deduction, but that’s besides the point of this post. Next time we’ll continue working with type deduction, but with the new auto feature this time.

decltype
This operator (yes, decltype is an operator) is a cousin of sizeof which will yield the type of an expression. Why do I say it’s a cousin of sizeof? Because it’s been in the compilers for a long time, only in disguise. This is because you can’t get the size of an expression without knowing it’s type, so even though it’s implementation has existed for a long time only now it’s available to the programmer.

One of it’s interesting features is that the expression with which you call decltype won’t be evaluated, so you can safely use a function call within a decltype, like this:

auto foo(int x) -> decltype( bar(x) ) {
return bar(x);
}

Doing this with, say, a macro, would get bar(x) evaluated twice, yet with decltype it will be evaluated only once. Any valid C++ expression can go within a decltype operator, so for example this is valid too:

What’s the type of A and B? What’s the type of A*B? We don’t care, the compiler will take care of that for us. Let’s look again at that example, more closely:

-> (delayed declaration) and decltype

Why bother creating a delayed type declaration at all and not just use the decltype in place of the auto? That’s because of a scope problem, see this:

// Declare a template function receiving two types as param
template <typename A, typename B>
// If we are declaring a multiplication operation, what's the return type of A*B?
// We can't multiply classes, and we don't know any instances of them
auto multiply(A x, B y)
// Luckily, the method signature now defined both parameters, meaning
// we don't need to expressly know the type of A*B, we just evaluate
// x*y and use whatever type that yields
-> decltype( x*y )
{
return x*y;
}

decltype
As you see, decltype can be a very powerful tool if the return type of a function is not known for the programmer when writing the code, but you can use it to declare any type, anywhere, if you are too lazy to type. If you, for example, are very bad at math and don’t remember that the integers group is closed for multiplication, you could write this:

int x = 2;
int y = 3;
decltype(x*y) z = x*y;

Yes, you can use it as VB’s dim! (kidding, just kidding, please don’t hit me). Even though this works and it’s perfectly legal, auto is a better option for this. We’ll see that on the next entry.

In the last two entries we worked on a wrapper object which allows us to decorate a method before or after calling (hello aspects!), or at least that’s what it should do when g++ fully implements decltypes and variadic templates. Our wrapper function looks something like this (check out the previous entry for the wrapper object):

After the example, we were left with three new syntax changes to analyze:

-> (delayed declaration)

decltype

auto

Let’s study the -> operator this time:

-> (delayed declaration)
This is the easiest one. When a method is declared auto (I’ve left this one for the end because auto is used for other things too) it means its return type will be defined somewhere else. Note that in this regard the final implementation differs from Stroustroup’s FAQ.

The -> operator in a method’s definition says “Here’s the return type”. I’ll paste the same simple example we had last time, the following two snippets of code are equivalent: